1. Field of the Invention
This invention relates generally to a disk drive unit, and is particularly but not exclusively applicable to a disk drive unit for rotating a micro floppy disk, for example, comprising a magnetic sheet having a diameter of 31/2 inches.
2. Description of the Prior Art
There is a prior art micro floppy disk drive unit, for example, disclosed in the specification of U.S. Pat. No. 4,445,155 which has been granted to the applicant of this invention.
As shown in FIGS. 5 to 7, a turntable 1 of the prior art comprises a rotatable base plate 2, a central pin 3, a drive pin 4, a supporting plate 5, a spring 6, a magnet 7 and a low friction sheet 8. The rotatable base plate 2 has a substantially circular shape and is fixed to the shaft 9 of a motor (not shown) by a force fit. The central pin 3 is provided at the top of the motor shaft 9.
The substantially C-shaped magnet 7 is made of an elastic magnet, and is adhered to the upper surface 2b of the base plate 2 around the boss 2a of the base plate 2. The circular low friction sheet 8 is made of non-magnetic material, and is adhered to the upper surface of the boss 2a so as to surround the central pin 3. The upper surface of the low friction sheet 8 functions as a disk mounting surface 10.
The substantially annular supporting plate 5 comprises a substantially U-shaped wide nonelastic portion 11 and a substantially U-shaped narrow elastic portion 12. An elastic projection 13 projects substantially from the center of the inside edge of the elastic portion 12, and the drive pin 4 is vertically secured to the elastic projection 13. The drive pin 4 comprises a bearing 14 and a bearing stand 4a. As shown in FIG. 9, a cylindrical head pin 4b is fitted, from above the bearing 14, into the hole of the bearing 14, and then fitted by a force fit into a hole of the bearing stand 4a that is fixed to the elastic projection 13 by upsetting, so that the bearing 14 is rotatable on the bearing stand 4a.
The supporting plate 5 is arranged on the lower face of the base plate 2, and the drive pin 4 projects upwards through a through hole 15 which is provided in the base plate 2. A fulcrum shaft 17 is fitted, from below the base plate 2, into a hole of the base plate 2, which is spaced away from the drive pin 4 substantially at a central angle of 90.degree. in the direction of reverse rotation of the turntable 1, and is vertically fixed to the base plate 2 by upsetting. Thus, the supporting plate 5 is rotatably secured to the base plate 2 through the fulcrum shaft 17. That is, the supporting plate 5 is horizontally rotatable on the fulcrum shaft 17 in the direction of the arrows b and c (FIG. 6). Further, since a U-shaped leaf spring 21 is mounted on the fulcrum shaft 17 to be interposed between the cylinder head 17a of the fulcrum shaft 17 and the supporting plate 5, the supporting plate 5 is also vertically rotatable in the direction of the arrows d and e (FIG. 10).
The tension coiled spring 6 is accommodated in a notch 18 provided on the outer periphery of the base plate 2, and stretched between a pair of spring stops 19 and 20 which are provided in the supporting plate 5 and the base plate 2, respectively, so that the drive pin 4 on the supporting plate 5 is urged in a direction departing from the center of the base plate 2, in the direction of the arrow c in FIG. 6.
A core 41, provided at the center of a micro floppy disk 40 (hereinafter described as a "disk") and made of stainless steel, has a central-pin insertion hole 42 of a substantially square shape in the central position thereof, and a drive-pin insertion hole 43 of a substantially rectangular shape in an eccentric position thereof. When the disk 40 is mounted on the disk mounting surface 10 of the turntable 1 as shown in FIG. 7, the central pin 3 and the drive pin 4 of the turntable 1 are inserted into the respective holes 42 and 43, and the core 41 is horizontally mounted on the disk mounting surface due to an attractive force of the magnet 7.
After the disk 40 is mounted, the turntable 1 is driven to rotate in the direction of arrow a, and the drive pin 4 fitted in the insertion hole 43 of the disk 40 is horizontally rotated on the fulcrum shaft 17 in the direction of the arrow b against a force of the spring 6. Next, the drive pin 4 presses the outer side 43b of the insertion hole 43 in a direction departing from the center of the turntable 1, in a direction of the arrow f, and presses the front side 43a of the insertion hole 43 in the direction of normal rotation, in the direction of the arrow a, so that the central pin 3 is pressed against two sides 42a and 42b of the central hole 42 which sides are adjacent to each other. Thus, the center of the disk 40 is accurately positioned by the central pin 3, and the disk 40 is rotated in the direction of the arrow a.
According to the disk drive unit of the prior art, the disk 40 is always accurately mounted on the predetermined position of the disk mounting surface 10 of the unit due to function of the drive pin 4, so that index positions (position reference signals) recorded on the disk 40 can be made constant.
However, as shown in FIG. 8 to 10, if the outer side 43b of the insertion hole 43 of the disk 1 mounts on the outer part of the upper surface 14a of the bearing 14 at the time when the disk 40 is mounted on the turntable 1, the core 41 of the disk 40 often inclines two sides with respect to the disk mounting surface 10. That is, as shown in FIG. 9, the core 41 comes in contact with the outer part of the upper surface 14a of the bearing 14 at a point A, and the disk mounting surface 10 at a point B which is situated at a short distance l.sub.1 from the point A. Thus, the core 41 inclines at a large angle .theta..sub.1 in a direction of the arrow X, and inclines at a large angle .theta..sub.2 in a direction of the arrow Y in FIG. 10, which is perpendicular to the arrow X.
As a result of an inclination of the angle .theta..sub.2, the upper end of bearing 14 engages with the front side 43a of the insertion hole 43, and the disk 40 is rotated by the turntable 1 in the direction of the arrow a, so that it becomes impossible to reproduce information from the disk 40.
To overcome the above problem, it is widely accepted that the turntable 1 is first rotated in a reverse direction of the arrow a' in FIG. 8 after the disk 40 is mounted, and then rotated in the normal direction of the arrow a in FIG. 8 so as to make the bearing 14 completely fit in the insertion hole 43 as shown in FIG. 7. However, it brings a result that a control circuit of the motor used for rotating the turntable 1 becomes complicated, and thereby, production costs will rise.